The present invention is directed to a very long bundled tensioning member for prestressing concrete with subsequent composite action within a tall structural member. The tensioning member is formed of a plurality of individual elements such as steel rods, steel wires or steel wire strands, and is insertable into a tensioning duct provided in the structural member with the duct in the form of a sheathing pipe or tube extending generally upwardly, vertically or diagonally, and accessible only at its upper end. The bundled tensioning member is anchored at one end within the tensioning duct and, subsequently, it is anchored by means of an anchoring disk. The invention is also directed to the method of installing the tensioning member.
Particularly in off-shore areas, it is often necessary to construct comparatively tall structural members of prestressed concrete, such as foundation elements for platforms or the like, extending from the ocean floor to above the surface of the water. Usually, such foundation elements are first constructed in a dock so as to float at a corresponding water depth by using a sliding construction, where the structural member sinks into the water in proportion to its height. In the course of such sliding construction, untensioned reinforcement and sheathing pipes for tensioning ducts can be installed, however, the tensioning members can only be introduced, tensioned and anchored for the full height of the structural member after it is completed.
Since the dimensions of such man-made structures has been optimized, that is, kept as small as possible for reasons of economy, a problem results in the installation of tensioning members, usually bundled tensioning members, for accommodating high loads in very narrow tensioning ducts, accessible only from one end. Usually, the other end of the duct is 50 m or more below the surface of the water. An effective anchorage must be provided within the narrow tensioning duct, since subsequent corrections are not possible. This anchorage must not take up more space than the tensioning member itself, because the sheathing pipe for the tensioning duct must have the same diameter along its full length to afford the introduction of the tensioning member.
The installation of steel tie rods or tensioning members in hollow spaces accessible from one end with the anchorage of the tension member at the inaccessible end is known in general in the installation of ground and rock anchors. To form the anchorage for securing the tension members, a hardenable material is injected into the base of the borehole, and the tension member is then tensioned from the opposite end and anchored by means of known anchoring devices used in prestressed concrete construction. The free length of the tension member between the anchorage and the anchoring device is freely extensive. Similar construction methods are known in anchoring structures, such as retaining walls, dam walls or the like, in the ground.
The known procedures for installing ground and rock anchors cannot be easily transferred to the production of prestressed concrete structures. Usually, there is enough space available in the ground for forming sufficiently large boreholes, even if costs increase with the diameter. Moreover, if such anchors prove to have insufficient bearing ability during subsequent monitoring, it is almost always possible to produce a new anchor. In underwater construction, which must satisfy the requirements of prestressed concrete structures, such replacement is not possible.
Accordingly, in the construction procedure mentioned above, it has been necessary to provide tensioning ducts for use in the erection of structural members with hairpin-shaped reversing points at their lower ends so that a tensioning member inserted in a tensioning duct can be tensioned at both ends from the upper end of the duct. To enable the insertion of the tensioning members into the tensioning ducts, large radius curvatures must be provided at the reversing points. Since the tensioning members must be arranged close to one another, they intersect in the region of the reversing points whereby causing a correspondingly great thickness of the structural member. Moreover, tensioning members in the form of steel wire strands have a long but limited length whereby only structural member height corresponding at most to half the length of the tensioning members can be achieved using such hairpin-shaped tensioning members. For larger structural member heights, intermediate joints in the tension members are needed and must also be hairpin-shaped.